Changes in plasma concentrations of immunoreactive ouabain in the tilapia in response to changing salinity: is ouabain a hormone in fish?

Ouabain, a cardiac glycoside and inhibitor of Na(+), K(+)-ATPase, is now believed to be a steroid hormone in mammals, involved in blood pressure and volume regulation and possibly acting as a natriuretic hormone. We have identified ouabain-like immunoreactivity in the plasma and tissues of a euryhaline teleost, the tilapia (Oreochromis mossambicus), by means of solid-phase extraction followed by a specific radioimmunoassay. Plasma concentrations of immunoreactive ouabain were 5-20pg/ml. Ouabain immunoreactivity was detected in all the tissues examined, with highest concentrations in the head kidney followed by intestine and body kidney. When the fish in fresh water were transferred to seawater, plasma osmolality increased significantly after 2, 4, 8, and 24h. Significant increases were observed in plasma ouabain immunoreactivity after 4 and 24h, and a significant correlation was seen between ouabain immunoreactivity and plasma osmolality. There was also a significant correlation between the plasma osmolality and cortisol concentrations. Upon transfer from seawater to fresh water, significant increases were seen in plasma cortisol after 4 and 8h and in immunoreactive ouabain after 4h. When the correlation was analyzed using all the data obtained during the two transfer experiments, plasma ouabain immunoreactivity and cortisol were significantly correlated with plasma osmolality, whereas there was a significant negative correlation between plasma prolactin and osmolality. A significant positive correlation was also seen between plasma cortisol and ouabain immunoreactivity. These results suggest that immunoreactive ouabain may be involved, together with cortisol, in the maintenance of hydromineral balance in the tilapia.

Induction of melatonin synthesis in Tetrahymena pyriformis by hormonal imprinting--a unicellular "factory" of the indoleamine.

Melatonin is present in Tetrahymena and its synthesis can be enhanced by pretreatment (imprinting) with melatonin. Two days after imprinting melatonin level is elevated in the cells and more elevated in the supematant. Such a minute quantity, as 10(-12) M melatonin for 1 hour is able to provoke imprinting, however the effect is more expressed using 10(-6) M. Maintenance in light conditions further elevated the amount of melatonin in the cells and supematant alike, related to the melatonin content of cells kept in darkness. The experiments call attention to the light-sensitivity of imprinting-provoked melatonin production in Tetrahymena and to the possibility of using this property for important physiological functions in higher grades of phylogeny.

Melatonin in the unicellular Tetrahymena pyriformis: effects of different lighting conditions.

Melatonin content in the cellular fraction and medium of Tetrahymena pyriformis GL cultures was measured at different time points of light and dark exposures. Tetrahymena produced, stored and secreted immunoreactive melatonin, which in displacement and HPLC studies, behaved like synthetic melatonin. There was not a continuous secretion of melatonin produced by the cells. In contrast to this, storage of melatonin was observed, which was more expressed in dark conditions. Prolonged light exposure suppressed melatonin production and secretion alike, however it did not block it completely.

The influence of anaesthesia and surgery on the circadian rhythm of melatonin.

BACKGROUND: Operations are typically associated with sleep and other circadian rhythm disturbances. The present study was set up to evaluate the influence of spinal and general anaesthesia associated with knee surgery on the circadian rhythm of melatonin, which has sleep inducing properties. Previously this context has been studied only in some invasive operations and it might be that general anaesthesia induces more disturbances on circadian rhythm of melatonin than operations done with patients awake. METHODS: The circadian secretion pattern of melatonin was monitored during the pre- and postoperative evenings, nights and mornings to clarify possible anaesthesia/surgery-induced changes in the nocturnal secretion of melatonin and in the phase of the melatonin rhythm. The study included 20 patients scheduled for minor orthopaedic operations. The patients were randomised to receive either spinal or general anaesthesia. Melatonin was measured from evening and morning saliva samples radioimmunologically. The nocturnal urine before and after surgery was radioimmunologically examined for 6-hydroxymelatonin sulphate. RESULTS: Melatonin secretion evaluated from the saliva samples was significantly diminished during the first postoperative evening as compared with that during the preoperative evening (P<0.001). There was also a significant decline of 26% (P<0.05) in postoperative 6-hydroxymelatonin sulphate excretion. There was no significant difference in melatonin secretion between the spinal and general anaesthesia groups. CONCLUSION: Our findings suggest that anaesthesia in conjunction with surgery acutely disturbed the normal circadian rhythm of melatonin by delaying the onset of nocturnal melatonin secretion.

Radioiodinated tyrosyl-ouabain and measurement of a circulating ouabain-like compound.

BACKGROUND: Assays for endogenous ouabain, a cardiac glycoside believed to be involved in blood pressure and volume regulation, are characterized by laboratory-specific plasma values that are measured by different assays. Because of this variability, our study focused on the development of a new (125)I-labeled ouabain derivative for RIA of high sensitivity. METHODS: We generated rabbit antisera against a ouabain-thyroglobulin conjugate. A tyrosylated ouabain derivative for radioiodination was synthesized using periodate and sodium cyanoborohydride reagents. RESULTS: Mass spectrometric analyses showed that the main product of the tyrosylating reaction was tyrosyl-ouabain (molecular mass, 702 Da). This was radioiodinated with Chloramine-T and used as a tracer in a RIA, which gave an assay detection limit of 5 pmol/L (4 ng/L), 2-100 times lower than that in the corresponding (3)H-RIAs and 2-20 times lower than ouabain ELISAs, making it possible to measure low plasma concentrations of immunoreactive ouabain. Different amounts of SepPak C(18)-extracted plasma samples displaced the (125)I-labeled tyrosyl-ouabain tracer at the same rate at which authentic ouabain was displaced. Plasma immunoreactive ouabain coeluted with authentic ouabain in two different HPLC conditions. Using the new RIA, we found plasma ouabain concentrations, assayed as immunoreactive equivalents, of 10.0 +/- 1.3 pmol/L in healthy women and 12.0 +/- 0. 9 pmol/L in healthy men (mean +/- SE; n = 10), as well as 41.2 +/- 9. 6 pmol/L in rats. The concentrations were 2-90 times lower than those previously reported using different assay methods. CONCLUSIONS: Our ouabain (125)I-RIA enables reliable measurements of low endogenous concentrations of a ouabain-like compound for both physiological and clinical purposes.

Radioimmunoassay of plasma ouabain in healthy and pregnant individuals.

Ouabain was recently isolated from human plasma, bovine hypothalamus and bovine adrenal in attempts to identify endogenous substances inhibiting the cell membrane sodium pump. A number of radioimmunoassays have been developed in order to study the clinical significance of ouabain. The results have been controversial with regard to the presence and chemical nature of plasma ouabain-like immunoreactivity. We have now measured ouabain in healthy and pregnant individuals using solid-phase extraction of plasma samples followed by a new radioimmunoassay with the extraordinary sensitivity of at least 2 fmol/tube (5 pmol/l). Plasma extracts, a previously isolated human plasma ouabain-like compound and bovine hypothalamic inhibitory factor displaced the tracer in parallel and eluted identically with ouabain in high-performance liquid chromatography. Plasma ouabain immunoreactivity was found to be much lower than reported previously: 12.6+/-1.3 pmol/l in healthy men (mean+/-s.e., n=20) and 9.4+/-0.7 pmol/l in women (n=14). In pregnant women (n=28) plasma ouabain concentration was 16.3+/-4.0 pmol/l during the first trimester, 18.8+/-4.3 pmol/l during the second trimester and 24.3+/-4.0 pmol/l during the third trimester (all P<0.01 compared with non-pregnant women). Plasma ouabain 3-5 days after the delivery was 13.6+/-1.1 pmol/l (n=10, P<0.05-0.01 compared with second and third trimesters). The pregnancy-related changes in the plasma concentrations of ouabain resembled those of cortisol. Therefore cortisol was measured from the same plasma samples and a significant positive correlation was found (r=0.512, P=0.006). The similar profiles of plasma ouabain and cortisol during pregnancy and their rapid decreases postpartum are consistent with the adrenal cortical origin of ouabain and also show that the secretions of these hormones are possibly under the control of same factors.

2,3,7,8-Tetrachlorodibenzo-p-dioxin alters melatonin metabolism in fish hepatocytes.

Pineal hormone melatonin is an important regulator of endocrine and circadian rhythms in vertebrates. Since liver is assumed to be the major organ in the metabolism of this indole hormone, we investigated the effect of the known Ah-receptor agonist, 2,3,7, 8-tetrachlorodibenzo-p-dioxin (TCDD) on melatonin metabolism in fish hepatocytes as well as the in vitro effect of melatonin on trout hepatic microsomal cytochrome P4501A (CYP1A) catalyst. Primary cell cultures of rainbow trout hepatocytes were exposed to [3H]melatonin (1 nM to 1 microM) alone and in combination with TCDD (50 pM) at 15 degrees C for 24 or 48 h. Analysis of melatonin and its metabolites in the culture medium and hepatocytes by HPLC revealed that about 96% of the added [3H]melatonin was metabolised after 24 h in both control and TCDD treated cultures. 3H-radioactivity was found mainly in the culture medium and less than 5% of the total 3H-radioactivity retained inside hepatocytes. Of the HPLC separated metabolites, one coeluted with 6-hydroxymelatonin and one unknown metabolite eluted after 6-hydroxymelatonin. In addition, two other metabolites were more water-soluble than 6-hydroxymelatonin and were considered to be conjugated products. Treatment of the hepatocytes with TCDD increased the amount of the major oxidated product, 6-hydroxymelatonin, about 2.5-fold after 24 h and 1.2-fold after 48 h exposure, respectively when compared with the control cultures. Whereas the amount of the unknown metabolite eluting after 6-hydroxymelatonin decreased about 1.3-fold after 24 h and 1.2-fold after 48 h exposure, respectively. Melatonin alone did not affect P4501A associated EROD-activity or CYP1AmRNA levels in the primary hepatocyte cultures. TCDD-treatment increased EROD-activity 3 to 5-fold and respective CYP1AmRNA content 6 to 14-fold, when compared with the control or melatonin-treated cultures. Furthermore, melatonin competitively inhibited EROD-activity in liver microsomes with a Ki value of 62.06+/-3.78 microM. The results show that TCDD alters metabolic degradation of melatonin in hepatocytes and suggest that P4501A may be an important P450 isoenzyme involved in oxidative metabolism of melatonin in fish liver.

Suppression of melatonin secretion by bright light in seasonal affective disorder.

Eleven patients with winter seasonal affective disorder and 10 healthy controls were exposed to light of 3300 lux for 5 min and for 1 hour respectively on consecutive evenings at 22:00 hours during winter and summer. In the winter, the measurements were undertaken both before and after the treatment with bright light for 2 weeks. In the summer, there was no treatment. Melatonin concentration in saliva and subjective sleepiness were measured at 22:00 and 23:00 hours on each test. There was no significant difference in the suppression of melatonin in response to the light tests between the patients and the controls. Exposure to light reduced the level of subjective sleepiness more among the patients compared to the control subjects. This reduction was not associated with the change in melatonin secretion nor the improvement in depressive symptoms.

Decrease in melatonin precedes follicle-stimulating hormone increase during perimenopause.

Melatonin, the hormone of the pineal gland, which in animal studies has been found to inhibit aging processes, is secreted in smaller amounts towards senescence. Menopause, an aging process in women, is known to be associated with typical changes in gonadotropin and sex steroid secretion. Our main objective was to study the possible role of melatonin in the hormonal regulation of menopause. This study focused on detailed changes in melatonin and follicle-stimulating hormone (FSH) secretion cross-sectionally in pre- to postmenopausal females. Special attention was paid to females aged around 50 years, which is the mean menopausal age. Seventy-seven healthy female volunteers aged 30-75 years were the subjects of this study. Melatonin was measured radioimmunologically from nocturnal urine collected between 20.00 and 08.00 h, and FSH and melatonin from blood samples taken at 0.900 h. Nocturnal urinary excretion of melatonin was found to decline significantly from premenopause to postmenopause. The youngest premenopausal women (age group 30-39 years) excreted the highest amounts of melatonin (21.2 +/- 2.2 pmol/h, mean +/- SEM, N = 17). In the age group 40-44 years the excretion declined by 41% (p < 0.05). The second significant decline (35%, p < 0.05) took place between the age groups 50-54 years and 55-59 years. A declining trend as a function of age was also seen in morning serum melatonin. Serum FSH rose sharply to high levels before the age of 50 (p < 0.01) and remained at a high level thereafter. Urinary melatonin correlated negatively with serum FSH (r = -0.32, p < 0.05). In conclusion, the inverse changes in melatonin and FSH secretion during the perimenopausal years, with the sharpest decline in nocturnal excretion of melatonin far before menopause, suggest that melatonin may be permissively linked to the initiation of menopause.

Ethanol decreases nocturnal plasma levels of thyrotropin and growth hormone but not those of thyroid hormones or prolactin in man.

Previous studies on the effects of ethanol on circulating pituitary hormones have been carried out mostly during daytime when the secretion of these hormones is generally at a nadir. Therefore, we studied the effects of ethanol on the nocturnal secretion of GH, PRL, TSH, and thyroid hormones (protocol I, nine healthy subjects, five women) and on the TSH and PRL responses to synthetic TRH (protocol II, healthy subjects, four women). Ethanol was given in doses of 0, 0.5 or 1.0 g/kg of BW(protocol I) and 0 or 1.0 g/kg (protocol II) and ingested po at 1900-1945 h. In protocol I, plasma GH rose from 0.6 +/- 0.2 microgram/L (mean +/- SE) at 2200 h to 25.0 +/- 4.3 micrograms/L at 0100 h in control subjects and was almost completely inhibited at 4.5 +/- 1.7 micrograms/L at 0100 h in subjects receiving 1.0 g/kg ethanol (P < 0.01). In subjects receiving 0.5 g/kg ethanol, the inhibition was also significant (P < 0.01), plasma GH being 8.2 +/- 2.5 micrograms/L at 0100 h. Plasma GHRH was measured after solid phase separation in RIA, but it did not show any ethanol-related changes. Plasma PRL exhibited a clear diurnal rhythm in control subjects and rose from 77 +/- 16 at 1800 h to 248 +/- 62 micrograms/L at 0700 h (P < 0.01). The plasma PRL profile was not affected by ethanol. Plasma TSH was 1.4 +/- 0.2 mU/L at 1800-2200 h and rose to 2.3-2.4 mU/L for 0100-0700 h (P < 0.001) in the control subjects. Ethanol 1.0 g/kg suppressed plasma TSH to 1.4 +/- 0.2 mU/L (P < 0.05 at 0100 h and P < 0.01 at 0200 h). According to the area under the curve analyses, the suppression in the nocturnal TSH was 32% in the 0.5 g/kg group and 45% in the 1.0 g/kg group (P < 0.05 for both cases). Circulating free or total T3 and T4 did not show any statistically significant changes that could explain the ethanol-induced inhibition in the nocturnal TSH peak. In protocol II, synthetic TRH (1 microgram/kg BW) was given intravenously, and blood samples were collected before, at 20 and 60 min. TRH significantly stimulated plasma TSH and PRL, but ethanol (1.0 g/kg BW) had no effect on these responses. In conclusion, small amounts of ethanol have unexpectedly great effects on nocturnal surges of TSH, and especially on those of GH, that are apparently mediated by suprapituitary mechanisms. On the other hand, ethanol did not affect the nocturnal PRL surge. These inhibitory effects of ethanol may have unfavorable effects on growth and metabolism in adolescent drinkers.

Effects of bright light on sleepiness, melatonin, and 25-hydroxyvitamin D(3) in winter seasonal affective disorder.

Sixteen patients with winter seasonal affective disorder and 13 healthy controls were exposed to 3300 lx of cool-white fluorescent light for either 1 hour or 15 min in the morning for 2 weeks during the winter. Subjective sleepiness, melatonin concentration in saliva, and serum 25-hydroxyvitamin D(3) concentration were measured before and after the 2-week trial as well as the following summer when the patients were well. There were no significant differences in the baseline values between the patients and healthy subjects. No significant differences in the outcome measures were observed in the patients or the controls in the two groups of each after the trial. The exposure to bright light resulted in a significant decrease in subjective sleepiness early in the evening in the patients but not in the control subjects. The reduction of depressive symptoms was associated with the decrease in subjective sleepiness but not with the changes in the melatonin or vitamin D concentrations.

Mechanism by which 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) reduces circulating melatonin levels in the rat.

We have previously shown that the prototype for halogenated aromatic hydrocarbons, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), diminishes serum melatonin concentration at the same dose in both the most TCDD-susceptible (Long-Evans, Turku AB; L-E) and the most TCDD-resistant (Han/Wistar, Kuopio; H/W) rat strain. The change developed within 24 h and persisted for at least 28 days after TCDD exposure; was independent of the time of day and was not associated with any morphological damage to the pineal gland. In the present study, we investigated the mechanism of this endocrine effect. Despite a 40-50% decrease in circulating melatonin levels, the pineal content of melatonin, serotonin and 5-hydroxyindole acetic acid remained unaltered and the rate-limiting enzyme of pineal melatonin biosynthesis, N-acetyltransferase, displayed only a relatively minor suppression in activity (30%) in TCDD-treated L-E rats. Likewise, TSDD did not influence the ability of pineal glands from L-E rats to synthesize and secrete melatonin in ex vivo or in vitro experiments. TCDD accelerated the disappearance of exogenous melatonin from the serum in both rat strains. This enhancement probably did not originate in the liver, because liver perfusion studies revealed that even control rat livers were capable of total melatonin clearance in spite of the fact that the melatonin concentration far exceeded physiological levels. Urine excretion of the normal main metabolite of melatonin, 6-hydroxymelatoninsulfate, was reduced by TCDD treatment in both strains. This was accompanied by an altered HPLC pattern of metabolites, especially in H/W rats. We conclude that TCDD decreases serum melatonin levels in rats by enhancing the peripheral, evidently extrahepatic, metabolism of the hormone.

Effects of exposure to morning bright light in the blind and sighted controls.

Seven blind subjects and 11 sighted controls were exposed to 3300 lux of cool-white fluorescent light for either 1 h or 15 min in the morning for 2 weeks during the winter. Serum 25-hydroxyvitamin D3 concentration, melatonin concentration in saliva, body temperature from the armpit, subjective sleepiness, and depressive symptoms were measured before and after the 2-week trial. The intervention resulted in a significant elevation in the concentration of melatonin at 21.00 hours in the healthy controls but at 23:00 hours in the blind subjects. The body temperatures measured were increased in the controls but decreased in the blind in the morning following the cessation of the intervention, and these opposite changes resulted in significant differences in the temperatures between the two groups. The decreases in the body temperature were associated with the increases in the levels of melatonin in the blind but not in the controls. Bright light administered in the morning decreased subjective sleepiness and improved mood in the healthy controls and in the blind subjects as well. The intervention had no effect on the levels of vitamin D in either of the two groups.

Ethanol induces a paradoxical simultaneous increase in circulating concentrations of insulin-like growth factor binding protein-1 and insulin.

The aim of this study was to examine the effect of acute alcohol intake on circulating concentrations of insulin, C-peptide, insulin-like growth factor (IGF) binding protein-1 (IGFBP-1), and plasma glucose levels. We measured these parameters for 12 hours after administration of 0, 0.5, or 1.0 g ethanol/kg body weight to nine healthy volunteers between 7:00 and 7:45 PM according to a randomized, double-blind, crossover design. Following a snack at 9:00 PM, plasma insulin (P < .05) and C-peptide (P < .01) concentrations were significantly increased at 10:00 PM in the 1.0-g group as compared with the control group. C-peptide to insulin molar ratios were significantly higher (P < .05) in both ethanol groups at 10:00 PM and 2:00 AM than in the control group. No differences were observed in plasma glucose levels between the three groups. Plasma IGFBP-1 levels showed a dose-dependent increase in the ethanol groups, and remained increased from 10:00 PM for 3 hours (P < .05 or less) at the lower dose and for 6 hours (P < .05 or less) at the higher dose. These observations indicate that ethanol-induced postprandial hyperinsulinemia is due to increased insulin secretion and that alcohol may increase hepatic insulin extraction. The lack of any effect on plasma glucose levels suggests that alcohol intake must be associated with decreased insulin sensitivity. Alcohol intake results in a paradoxical increase in peripheral concentrations of IGFBP-1 despite simultaneous hyperinsulinemia. This implies that ethanol has a direct stimulatory effect on hepatic IGFBP-1 synthesis.

Delayed pro-opiomelanocortin activation after ethanol intake in man.

To elucidate the effect of ethanol on the secretion of ACTH and beta-endorphin (BE) as the representatives of the pro-opiomelanocortin (POMC) system, as well as cortisol as the hypophyseally regulated peripheral hormone, we measured concentrations of serum ethanol and plasma ACTH, BE, and cortisol at 1- to 4-hr intervals for 12 hr after administration of 0.5 and 1.0 g ethanol/kg of body weight and placebo drinks between 1900-1945 hr to nine healthy volunteers according to a double-blind, cross-over design. Plasma ACTH, BE, and cortisol showed an expected diurnal rhythm with the highest levels at 0700 hr. Intake of ethanol had no statistically significant effects on plasma ACTH up to 0700 hr in the morning. The higher dose caused increased levels of BE at 0100 hr and both doses at 0200 hr. Plasma cortisol at 0400 hr was higher in subjects receiving 1.0 g ethanol/kg than in those receiving placebo (p < 0.05). Our present observation that plasma ACTH was unchanged after ethanol intake, but plasma BE was increased at 0100-0200 hr may be due to the fact that our BE antiserum cross-reacts with beta-lipotropin, which has a considerably longer half-life than ACTH or BE, and also to the long sampling interval. Thus, the POMC system may have been stimulated after ethanol intake. The nocturnal rise of plasma cortisol levels at 0400 hr, 2-3 hr after the peak in plasma BE, may be caused by the increased secretion of POMC. Because the ethanol dose of 1.0 g/kg body weight stimulated the POMC system but the 0.5 g/kg body weight did not, we conclude that higher ethanol doses induce increases in stress hormone secretion.

Ethanol decreases nocturnal plasma levels of atrial natriuretic peptide (ANP 99-126) but not the N-terminal fragment of pro-atrial natriuretic peptide (ANP 1-98) in man.

1. The aim of this study was to elucidate the role of atrial natriuretic peptides in the regulation of water and electrolyte balance after alcohol intake. To this end we measured the plasma concentrations of ethanol, atrial natriuretic peptide 99-126 and the N-terminal fragment of pro-atrial natriuretic peptide (atrial natriuretic peptide 1-98), serum osmolality and serum sodium concentration, and urine output, urine osmolality and urinary sodium excretion for 12 h after administration of ethanol (0, 0.5 and 1.0 g body weight/kg) and placebo drinks to nine healthy subjects according to a double-blind cross-over design. 2. Intake of ethanol (at 19.00-19.45 hours) inhibited the nocturnal increase in the plasma atrial natriuretic peptide 99-126 level dose-dependently (P < 0.05), but had no effect on the plasma atrial natriuretic peptide 1-98 level. Serum osmolality and serum sodium concentration were elevated dose-dependently for 2-5 h after the ethanol intake. Urine volume increased after the higher ethanol dose (net loss of 0.6 litre of water). 3. Since the plasma atrial natriuretic peptide 1-98 level was not changed after ethanol intake, we propose that the alcohol-induced inhibition of the nocturnal rise in the plasma atrial natriuretic peptide 99-126 level is not caused by an inhibition of release, but may rather reflect an increased peripheral elimination of atrial natriuretic peptide 99-126.

Cardiopulmonary and behavioral responses to computer-driven infusion of detomidine in standing horses.

Cardiopulmonary and behavioral responses to detomidine, a potent alpha 2-adrenergic agonist, were determined at 4 plasma concentrations in standing horses. After instrumentation and baseline measurements in 7 horses (mean +/- SD for age and body weight, 6 +/- 2 years, and 531 +/- 48.5 kg, respectively), detomidine was infused to maintain 4 plasma concentrations: 2.1 +/- 0.5 (infusion 1), 7.2 +/- 3.5 (infusion 2), 19.1 +/- 5.1. (infusion 3), and 42.9 +/- 10 (infusion 4) ng/ml, by use of a computer-controlled infusion system. Detomidine caused concentration-dependent sedation and somnolence. These effects were profound during infusions 3 and 4, in which marked head ptosis developed and all horses leaned heavily on the bars of the restraining stocks. Heart rate and cardiac index decreased from baseline measurements (42 +/- 7 beats/min, 65 +/- 11 ml.kg of body weight-1.min-1) in linear relationship with the logarithm of plasma detomidine concentration (ie, heart rate = -4.7 [loge detomidine concentration] + 44.3, P < 0.01; cardiac index = -10.5 [loge detomidine concentration] + 73.6, P < 0.01). Second-degree atrioventricular block developed in 5 of 7 horses during infusion 3, and in 6 of 7 horses during infusion 4. Mean arterial blood pressure increased significantly from 118 +/- 11 mm of Hg at baseline to 146 +/- 27 mm of Hg at infusion 4. Similar responses were observed for mean pulmonary artery and right atrial pressures. Systemic vascular resistance (baseline, 182 +/- 28 mm of Hg.ml-1.min-1.kg-1) increased significantly during infusions 3 and 4 (to 294 +/- 79 and 380 +/- 58, respectively). (ABSTRACT TRUNCATED AT 250 WORDS)

Ethanol inhibits melatonin secretion in healthy volunteers in a dose-dependent randomized double blind cross-over study.

To elucidate the effects of alcohol on pineal rhythmicity, ethanol was administered in the evening in amounts usually consumed during social ingestion to nine healthy volunteers in a double blind, cross-over study. Plasma concentrations of melatonin, catecholamines (norepinephrine and epinephrine), and ethanol were measured by RIA, high pressure liquid chromatography, and gas chromatography before and for 12 h after the administration of 0, 0.5, and 1 g ethanol/kg wt. Plasma melatonin and catecholamines displayed expected diurnal rhythms, with peak values at 0300-0400 h for melatonin and trough values at 0100-0400 h for catecholamines. Intake of ethanol between 1900-1945 h inhibited the nocturnal melatonin secretion dose-dependently during the first half of the night, with no changes in urinary excretion of melatonin. The inhibition was 41% (P < 0.05) from the control at midnight for both ethanol doses, 33% (P < 0.05) at 0100 h, and 18% (P < 0.05) at 0200 h for the higher dose. In addition, the higher dose of ethanol increased plasma norepinephrine levels at 2000 and 2200 h (P < 0.01) until 0400 h (P < 0.05). Taking into account the involvement of melatonin in the regulation of sleep and diurnal rhythms, we suggest that ethanol-induced suppression of nocturnal melatonin secretion and an increase in noradrenergic activity may be closely associated with disturbances in sleep and performance.

Noradrenergic inhibition and alpha 2-adrenergic stimulation of melatonin secretion in the pigeon.

Adrenergic regulatory mechanisms of melatonin synthesis and secretion were studied in the pigeon in vivo. Late-afternoon intraperitoneal injection of noradrenaline (NA; 1 mg/kg) resulted in a significant decrease in plasma melatonin levels in 3 h. The same effect was seen after phenylephrine treatment (1 mg/kg i.p.), indicating that an alpha 1-adrenergic mechanism may mediate the inhibition. Propranolol treatment had no effect on plasma melatonin levels, supporting this concept. Detomidine (1 mg/kg i.p.), an alpha 2-adrenergic agonist, increased melatonin levels. This stimulatory effect was blocked by yohimbine, an alpha 2-adrenergic antagonist. However, yohimbine alone had no effect on the plasma melatonin levels, suggesting that alpha 2-adrenergic transmission is not primarily responsible for the nocturnal stimulation of melatonin synthesis and secretion in the pigeon.